1. Field of the Invention
The present invention relates to a liquid crystal panel and a projection display which uses the liquid crystal panels as light valves.
2. Description of the Prior Art
Liquid crystal panels have been developed intensively because it has many advantages such as light weight and compactness. However, there remain many problems. For example, it is difficult to enlarge the size of display area. Then, a projection display has attracted attention wherein a display image of a small size liquid crystal panel is magnified by projecting it with a projection lens or the like on a screen. A liquid crystal panel to be used in a projection television has to be compact. Otherwise the size of projection lens increases and the system size also becomes large, and this results in a high cost and bad portability.
A commercial liquid crystal projection television uses a twisted nematic liquid crystal panel which uses optical rotation characteristic of liquid crystal. The liquid crystal projection television needs a polarizer and a analyzer. If an aperture ratio of pixels is assumed to be 100% and a quantity of light is assumed to be one, a quantity of light transmitting the polarizer is say 40% and transmittance of the analyzer is say 80%. Then, a total transmittance becomes 0.4 * 0.8=0.32. Thus, only about 30% of light can be used effectively. Therefore, an image in a display has low brightness. Further, almost all light loss in the polarizer and the analyzer are transformed to heat and the panel is heated by radiation. Because a quantity of light incident on a liquid crystal panel amounts to about a few ten thousand luxes, the polarizers and the panel are heated to a high temperature and this deteriorates them in a short time.
Then, a projection liquid crystal display is proposed wherein a polarizer and an analyzer are not needed. For example, a polymer/resin composite liquid crystal or the like can be used for such a liquid crystal display. A liquid crystal/resin composite is a composite of a liquid crystal component and a resin component, and the liquid crystal component exists in the matrix of the resin (polymer) component. Besides the polymer/resin composite liquid crystal panel, a light beam writing type panel (refer, for example, Japanese Patent laid open Publication 2-93519/1990) and a micro mirror panel (for example, U.S. Pat. No. 4,566,935) do not need a polarizer and an analyzer.
FIG. 1 shows an example of a projection display. A light emitted from a collimation optical system 321 is guided by a color separation and synthesis optical system wherein a white light is separated into red, green and blue lights and they are synthesized again after transmitting liquid crystal panels. The system comprises mirrors 322a, 322b, three polymer dispersion liquid crystal panels 323a-323c, and dichroic mirrors 324a-324d. The dichroic mirrors 324a-324d are set to have an incident angle of 45.degree.. Then, the light is projected by a projection lens 325 through an aperture 326 onto a screen 19.
However, this projection display has a disadvantage that spectral characteristic is deteriorated in the color separation and synthesis optical system, because dichroic mirrors or dichroic prisms are used therein for white or random color. An optical multi-layer film made of dielectric layers is provided at a color separation plane of a dichroic mirror or prism. It is known that a difference in a spectral characteristic between P and S polarization lights increases with increasing incident angle at the optical multi-layer film. Then, an average of the P and S polarization lights is generated by the dichroic mirrors or prisms. Therefore, a sharp frequency cut-off cannot be realized. This means that color purity of outgoing light is deteriorated than that of incoming light.
On the contrary, a projection display which uses a twisted nematic liquid crystal has a better spectral characteristic because only P or S polarization light is transmitted through the dichroic mirrors or prisms. Therefore, even if polarization dependence of light exists, sharp color separation characteristic is realized and a hue in a projected image becomes better.
FIG. 2 shows a projection polymer dispersed liquid crystal display disclosed in U.S. Pat. No. 5,245,449 which solves this problem. In the projection display, the incidence angle at the dichroic mirror is restricted between 15.degree. and 35.degree. to decrease deterioration of hue. However, the projection display has disadvantages due to the incidence angle of 35.degree. or less.
A first disadvantage is that a back focus of the projection lens becomes long. That is, distances between the projection lens and the panels become longer than the counterparts shown in FIG. 1. Then, the design of the projection lens becomes hard. Further, the dichroic mirrors 324 have larger areas, and this increases their costs. A second disadvantage is that a system size becomes larger. If two systems shown in FIGS. 1 and 2 are compared with each other, it is clear that an area required by the color separation and synthesis optical system becomes larger in the display panel shown in FIG. 2. The size of the optical system is directly reflected in the system size, and a larger system size decreases an advantage of a projection liquid crystal display or its compactness.